Urethane polymers or polyurethanes are a large family of polymers with widely varying properties and uses, all based on the reaction product of an organic isocyanate with compounds containing a group with an active hydrogen, such as a hydroxyl group. Polyurethane polymers are generally classified into two broad categories. A. foam or urethane foam, and B. elastomers or polyurethane elastomers. Polyurethane foams are polyurethane polymers produced by the reaction of polyisocyanates with a hydroxyl group from a polyol and a polymerization catalyst, in the presence of water and/or an auxiliary blowing agent, such as monofluorotrichloromethane, which allows the polymeric mass to expand into a cellular mass upon reaction. In preparing a non-cellular polyurethane elastomer, no blowing agent or mechanism for producing gas which would lead to cell development should be present. Therefore, the polymer is produced by the reaction of the isocyanate with a hydroxyl group to form urethane linkages in the presence of a polymerization catalyst.
Polyurethane elastomers have been widely used in a variety of applications. They have been used as protective coatings, in the insulation of electrical elements, as caulks, sealants, gaskets, etc. Because of favorable rheology of an elastomer formulation, they can be used to cast intricate forms such as found in the toy industry. They have also been widely used in the preparation of sporting goods, fabric coatings and shoe soles wherein the cured urethane elastomer comes in repeated intimate contact with human beings. The prior art catalysts used to prepare non-cellular elastomers frequently contained toxic mercury and lead compounds and the toxicity was carried over into the cured elastomer. If less toxic organo-tin compounds are employed as catalysts, elastomers having physical properties less than optimum are often obtained.
There are several patents relating to various catalysts for reacting isocyanates with polyether polyols. U.S. Pat. No. 3,245,957 to Hendersinn et al. describes a process for reacting an isocyanate with an active hydrogen compound in the presence of an antimony containing catalyst.
U.S. Pat. No. 3,203,932 to Frisch et al. relates to a process for preparing urethane-urea elastomers using metal organic catalysts such as lead, cobalt and zinc naphthenates.
U.S. Pat. No. 4,468,478 to Dexheimer et al. discloses polyurethanes prepared from polyoxyalkylenes containing alkali metal or alkaline earth metal catalyst residues chelated with benzoic acid derivatives.
U.S. Pat. No. 3,714,077 to Cobbledick et al. relates to a urethane foam catalyst system consisting of a combination of polyol-soluble organic stannous compounds with polyol-soluble organic bismuth and/or antimony compounds with certain sterically hindered tertiary amines.
U.S. Pat. Nos. 3,801,532, 4,000,103, 4,000,104 and 4,001,165 to Olstowski disclose rapid-setting polyurethanes prepared from diols and polyfunctional isocyanates using organo-metal compounds of tin, zinc, lead, mercury, cadmium, bismuth, cobalt, manganese, antimony and iron, such as stannous octoate, manganese octoate, lead octoate, and dibutyl tin dilaurate.
U.S. Pat. No. 4,584,362 to Leckart et al. relates to polyurethane elastomers prepared utilizing as the sole catalyst therein a bismuth salt of a carboxylic acid having from 2 to 20 carbon atoms.
U.S. Pat. No. 4,452,829 to Smith discloses a sprayable polyisocyanate composition prepared by reacting MDI with a triol and a diol utilizing potassium octoate as a trimerization catalyst and an amine-type heat-activated catalyst and, optionally, a tetravalent-tin urethane-type catalyst to provide a solid, coating or foam.
Non-cellular polyurethanes are also used in plywood-patch applications to fill crevices, voids and other imperfections that occur during the manufacture of plywood. Polyurethanes are well suited for such applications because the isocyanates thereof have a natural affinity for wood. Typically, these compositions are applied in an assembly line fashion, thereby requiring a rapid cure at room temperature (generally less than one minute).
Currently, many formulations for this application employ organo-lead catalysts. However, concern over their toxicity has spurred the search for effective, non-toxic, alternative catalysts. Organo-mercury catalysts are too sluggish and are also toxic. Organotins and tertiary amines have a propensity to react with water causing foam formation which would affect the adhesion of the patch to the plywood and the mechanical properties, e.g. hardness, of the patch. As indicated by A. R. Leckart and L. S. Slovin in their article "New Catalyst for Two-Component Elastomer Systems," Journal of Elastomers and Plastics, vol. 19, pages 313-324 (1987), organo-bismuth catalysts appear to hold some promise in this regard, but are about 3 to 5 times more expensive than lead-based catalysts. Additionally, use of organo-bismuth catalysts in plywood-patch compositions as a sole catalyst is not desirous due to the fast rate of gelation achieved which adversely affects the flowability of the precured composition through the delivery equipment and onto the panel.
Organo-potassium catalysts are a very active species of catalyst, but they have a great affinity for water (hygroscopic) and also a propensity to react with water causing foam formation. Additionally, during their formation, water is produced which is typically not removed due to handling considerations. When the water of reaction is removed, such catalysts become too viscous and even become solid as in the case of potassium octoate, thereby presenting a handling problem. As such, organo-potassium catalysts are typically relegated to the production of foams or applications where foaming is not detrimental thereto. However, there is presently a commercially available plywood-patch composition which appears to be utilizing a potassium-based catalyst believed to be potassium octoate. As to be expected, the patch exhibits slight foam formation due to water contained therein which is sufficient to significantly impair the properties thereof, particularly adhesion, hardness and cure time.